"Mere" bone? Are you joking? And what source would you accept? A scientific notebook from an alien paleontologist of 100 million years from now that happened to fall through a wormhole and into my lap? What source do you expect me to find?
I don't know if I even understand your question
. You want me to demonstrate somehow that bones - which fossilize and have demonstrably lasted across millions of years, even hundreds of millions of years - will not
be outlasted by:
- rebar-reinforced concrete - which already looks like rocks because concrete is just a mix of cemented rocks. After millions of years it's still going to look just like a rock. Most people already have a hard time telling a piece of concrete from rock now. And the rebar will just rust and react with everything over millions of years. Rebar-reinforced concrete isn't going to last across millions of years.
- cast iron - which would just look like a lump of rusted iron ore after millions of years in the rock cycle, if it remained in one lump, which is highly unlikely, since it is brittle and will break eventually. Even if a lump of "cast" iron (it wouldn't be "cast" anymore after millions of years) were found by future archaeologists or paleontologists, what would that prove?
- titanium, which oxidizes eventually at high temperatures, too; it's not indestructible, you know, and let's face it, it's not the most common material in our civilization. Even if a titanium bike frame lasted intact across millions of years, what do you think the odds of it being found are?
If you'd read the meaning of my post, you will discover that I meant that it is almost impossible that there will come a day when human bones are the only evidence left of human civilisation. We have created structures designed to last for a very long time. The pyramids, the Great Wall of China. they may not survive in a form we can recognise, but an investigation will reveal non-natural origins. Other examples could include the vast road networks across many countires, or the foundations built to support the many absolutely huge buildings that we have constructed. Also dams. Even if they are reduced to apparent rocks, an investigation will show that they are very unusual rocks.
The fact is that any space faring culture will leave behind evidence of that culture in some form.